Update contributing instructions
Update contributing instructions

Add a link to the thoughtbot code of conduct.

prahlad37/pick,GBuella/pick,calleerlandsson/pick,thoughtbot/pick,prahlad37/pick,thoughtbot/pick,prahlad37/pick,thoughtbot/pick,DBOTW/pick,DBOTW/pick,DBOTW/pick,temochka/pick,calleerlandsson/pick,temochka/pick,prahlad37/pick,GBuella/pick,thoughtbot/pick,calleerlandsson/pick